Industrial process control systems are used to monitor and control industrial processes that produce or transfer fluids or the like. In such systems, it is typically important to measure “process variables” such as temperatures, pressures, flow rates, and others. Process variable transmitters are used to measure such process variables and transmit information related to the measured process variable back to a central location such as a central control room.
A process variable transmitter generally includes or is coupled to a transducer or sensor that responds to a process variable. A process variable generally refers to a physical or chemical state of matter or conversion of energy. Examples of process variables include pressure, temperature, flow, conductivity, pH and other properties. Pressure is considered to be a basic process variable that can be used to measure flow, level and even temperature.
In order to measure a fluid flow, it is often necessary to determine a number of process variables, such as process fluid temperature, process fluid static or line pressure, and process fluid differential pressure across a partial obstruction, such as an orifice plate or the like. In such instances, multivariable transmitters are commonly used to measure and monitor a plurality of process variables in order to provide calculated parameters, such a process fluid flow. Such calculated parameters are useful relative to various industrial process fluids, such as slurries, liquids, vapors and gases of chemical, pulp, petroleum, gas, pharmaceuticals, food and other fluid-type processing plants.
Multivariable process fluid transmitters generally include a differential pressure sensor as well as a line pressure sensor and/or a process fluid temperature sensor. The differential pressure sensor responds to a difference in pressure between two process fluid inputs. The line pressure sensor responds to the absolute or gauge pressure in one of the fluid inputs. The process fluid temperature sensor responds to the temperature of the process fluid with an electrical indication, such as a voltage or resistance, that is related to the temperature of the process fluid.
In multivariable process fluid transmitters that include a differential pressure sensor, such transmitters typically include a pair of isolator diaphragms that are positioned in the process fluid inlets and isolate the differential pressure sensor from the harsh process fluids being sensed. Pressure is transferred from the process fluid to the differential pressure sensor through a substantially incompressible fill fluid carried in a passageway extending from each isolator diaphragm to the differential pressure sensor.
High static pressure environments can provide significant challenges for process fluid transmitters. In some cases, the bolted connection between the process fluid flange and the process variable transmitter base typically cannot seal at such high pressures due to stress limitations of the bolts and deformable seals used therebetween. When the seal is deformed or otherwise disrupted, process fluid may leak from the coupling. Currently, multivariable process fluid transmitters are not able to operate in environments rated to high line pressures, such as 15,000 psi. Thus, current multivariable devices are not generally suitable for some process environments such as subsea use. Accordingly, in such environments, when a flow measurement or other similar measurement is desired which requires multiple process variables, multiple process fluid transmitters, such as two and sometimes three process fluid transmitters are required. Providing such transmitters involves considerable expense. Thus, for growing high-pressure markets, such as subsea oil and gas wells, it is desirable to provide a multivariable process fluid transmitter that is suitable for such environments and can provide all requisite process variables using a single device.